1. Field of the Invention
The present invention relates to a load drive circuit with a current bidirectional detecting function.
2. Description of the Related Art
A current loop of a load drive circuit configured to unidirectionally supply a current from a DC power supply to a load to drive the load is typically provided with a switching element configured to control the current supplied to the load. In addition, since it is necessary to detect and monitor the current flowing through the switching element to the load for the purpose of overcurrent protection of the switching element, the load drive circuit has a function of detecting the current flowing to the load.
For example, Japanese Laid-Open Patent Application Publication No. 1989-83156 describes that chopper control is carried out by a field-effect transistor (hereinafter referred to as a “FET”) as the switching element in a chopper control system of a DC electric motor applied to an electrically-assisted power steering system of an automobile. Moreover, FIG. 1 of Japanese Laid-Open Patent Application Publication No. 1989-83156 discloses the configuration of a current detecting circuit configured to detect the current flowing to the DC electric motor (load). Hereinafter, a conventional load drive circuit with a unidirectional current detecting function shown in FIG. 10 that is FIG. 1 of Japanese Laid-Open Patent Application Publication No. 1989-83156 will be explained.
The conventional load drive circuit with the unidirectional current detecting function shown in FIG. 10 carries out the chopper control of a load 52 based on a chopper control FET (hereinafter referred to as a “main FET”) 53. The load 52 is constituted by a DC electric motor driven by a DC power supply wire 51, such as a battery. The main FET 53 is turned on and off by a gate signal supplied from a control device, not shown. Moreover, the conventional load drive circuit with the unidirectional current detecting function shown in FIG. 10 operates such that a current detecting FET (hereinafter referred to as a “sub FET”) 54 detects a current I1 which flows during the chopper control of the load 52. Specifically, the sub FET 54 is turned on and off at the same time as the main FET 53 by the gate signal supplied to the main FET 53. Moreover, a differential amplifier 55 operates to supply a source current I2 of a predetermined value to the sub FET 54 such that a source voltage V1 of the main FET 53 applied to a non-inverted input terminal and a source voltage V2 of the sub FET 54 applied to an inverted input terminal become equal to each other.
Here, if an ON resistance of the main FET 53 is (1/n) times an ON resistance of the sub FET 54, the relation between the current I1 flowing through the main FET 53 and the current I2 flowing through the sub FET 54 is shown by the following formula.I2=(1/n)·I1  (1)
Moreover, in a case where a resistance value of a resistor 56 is denoted by R, and a voltage drop of the resistor 56 is denoted by V1, the relation between V1 and I2 is shown by the following formula.V1=R·I2  (2)
Therefore, based on Formulas (1) and (2), the relation between V1 and I1 is shown by the following formula.V1=(1/n)·R·I1  (3)
It is clear from Formula (3) that the current I1 flowing to the load 52 can be indirectly detected by detecting the voltage drop VI of the resistor 56. Moreover, when the voltage drop VI of the resistor 56 exceeds a predetermined threshold Vth, that is, when the current I1 corresponding to the voltage drop VI exceeds a threshold Ith shown by Formula (4) below, the overcurrent protection is realized, that is, the main FET 53 is forcibly turned off.Ith=n·(Vth/R)  (4)